test: group examples into per-category folders

Move examples/*.sx and their expected/ snapshots into per-category
subfolders (examples/<category>/...). Folder = leading filename token,
with ffi-objc/ffi-jni kept whole; filenames are unchanged. The corpus
runner and LSP sweep now discover each category's expected/ dir, while
issues/ stays flat. Example 1058's repo-root-relative companion import
is made file-relative. Path strings embedded in 164 snapshots were
regenerated (path-only changes). Test-layout docs in CLAUDE.md updated.

examples/protocols/expected/0411-protocols-impl-duplicate.stderr

@@ -0,0 +1,5 @@
+error: duplicate xx conversion from 'i64' to 'Wrap': impls in examples/protocols/./0411-protocols-impl-duplicate-impl-a.sx and examples/protocols/./0411-protocols-impl-duplicate-impl-b.sx
+  --> examples/protocols/0411-protocols-impl-duplicate.sx:23:17
+   |
+23 |   w : Wrap = xx 7;
+   |                 ^